When making blood tests in the field, it is a desire to perform such tests with simple but reliable apparatus that can be handled even by relatively untrained personnel. Still, there exists the requirement that a blood sample shall be taken and handled under strict hygienic conditions, and that neither the sample itself or residues thereof, nor diluting or flushing liquids used when testing the sample shall risk to be contacted by humans. Thus, there shall be no waste matter and all contaminated material shall remain within the apparatus.
It is known in the state of art to count blood cells by causing a volume of diluted blood sample to pass a so-called capillary, i.e., an extremely small hole, generally in a ruby, the hole having a diameter considerably larger than the size of a blood cell, typically 80 μm. A voltage is applied over the capillary, and, when a blood cell passes through the hole, the electrical resistance changes. This is because the cells can be regarded as insulators. Each change in resistance can be detected by suitable electronic equipment, and the sum of all changes detected corresponds to the number of blood cells having passed through the capillary. In order to obtain the concentration of cells in the original sample, the concentration of cells in the diluted sample is multiplied by the dilution factor, typically 1:40000 when counting of red blood cells (RBC) is concerned. It is obvious, that measuring of sample volumes and dilution liquid volumes must be performed in an accurate and repeatable way such that not only a correct degree of dilution can always be guaranteed but also a thorough and uniform mixing of the two volumes is ensured.
A disposable sampling device for an apparatus for counting particles contained in a liquid, such as blood cells in a blood sample, is known from WO 99/01742. This device is capable of making one diluting step.
A blood testing apparatus for performing dilution of a small defined volume of blood sample contained in a capillary tube is described in U.S. Pat. No. 6,284,548. The dilution involves a pre-dilution step and a final dilution step.
A device for diluting and mixing a liquid sample, such as a blood sample for performing a CRP test, is described in WO 01/75416. The sample is contained in a capillary tube and is mixed in a first step with a diluting agent to provide a diluted sample. In a second step, a third medium, such as antibodies, may be mixed with the diluted sample.
Even if some of the prior art devices are capable of making two dilutions, none of them is capable of making two simultaneous dilutions to different dilution ratios, which is desirable in order to perform, e.g., simultaneous counting of white and red blood cells.
A disposable apparatus for use in blood testing, having one of the present co-inventors as single inventor, is described in SE 0103877-7 filed 21 Nov. 2001 and unpublished at the date of filing the present application. It presents one solution to the problem of providing such apparatus allowing simultaneous dilution of a blood sample to two defined dilutions ratios. It is also capable of retaining all contaminated material within itself.
This prior apparatus comprises a block-shaped housing having a first and a second receptacle; a first and a second cylinder, each having a piston moveable therein and each containing a defined volume of a diluent; a valve including a valve body having three valve body channels extending therethrough and being positionable in three distinct positions. In one position the receptacles are put in simultaneous communication with one each of the cylinders through pairs of the channels.
One of the receptacles, as a first means for receiving a blood sample, is adapted to receive a blood sampling capillary tube.
Although fulfilling the objectives stated, this apparatus presents some inconveniences. One relates to the manufacture of the block-shaped housing, which is expensive and complicated due to its various cylinders, and makes it unsuited for injection moulding. Another relates to the use of cylinders as means for containing the diluent, and pistons movable within the cylinders to displace the diluent. It manifests itself particularly during air transportation when the pistons tend to move uncontrolled due to a varying surrounding air pressure.